1. Field of the Invention
This invention relates generally to a sensing device which, in conjunction with a rotor that is connected to a rotating body, is used to determine the velocity of the body, and more particularly to a sensing device that can be used with rotors of different sizes or configurations.
2. Description of the Prior Art
Numerous sensing devices have been developed for determining the angular velocity of a rotating body. Most of these devices employ an electromagnet which produces a magnetic field. This field is interrupted by teeth, apertures, or ripples on a rotor which is synchronized with and/or connected to the rotating body. Such interruptions produce a magnetic flux change which is used to induce an output signal with a frequency proportional to the angular velocity of the rotating body. Since these devices are dependent solely on flux change for generation of an output signal, it is imperative that the flux change resulting from the rotation of the rotor be significantly larger than any flux change resulting from axial movement of the rotor relative to the sensing device. In essence, a high signal-to-noise ratio is required to obtain an accurate indication of the angular velocity of the rotating body.
A typical application requiring such a high signal-to-noise ratio is an anti-skid system for automotive and truck vehicles. In such systems, the sensing device generates a signal with a frequency that is proportional to the angular velocity of the vehicle wheel. This signal is then utilized in an electrical control circuit which, through mechanical means, regulates the application of the vehicle brakes. In this type of system, it is imperative that the signal generated by the sensing device be an accurate representation of the wheel velocity so that a skid condition can be detected and immediately corrected. Such accuracy is dependent upon a high signal-to-noise ratio.
Obtaining a high signal-to-noise ratio has been a particularly troublesome problem with these sensing devices, especially in an anti-skid system environment. Some axial movement of the rotor relative to the sensing device is always present due to inherent dimensional variations between the wheel, axle, rotor and sensing device and vibratory movement of these components. Various approaches have been used to overcome or minimize this problem, such as by using components manufactured to close tolerance or by employing elaborate sensor mounting techniques. These approaches are very costly and have not been satisfactory.
Another problem with these sensing devices is that, in general, they can only be used with one configuration of rotor tooth or aperture spacing. Since the pole pieces in the sensing devices are spaced relative to each other so as not to be in register simultaneously with the rotor teeth or apertures, the spacing between pole pieces is directly related to rotor tooth or aperture spacing. Thus, if a different rotor diameter is used which has a different tooth or aperture spacing, another sensing device, with the proper pole spacing, must be installed. Because of this, a multiplicity of sensing devices with different pole piece spacings is required to cover a range of rotor diameters. Such a family of sensing devices is costly to tool for production and results in a higher unit manufacturing cost than if one sensing device could be used for a plurality of rotor tooth or aperture spacings.
Because of the aforementioned problems, it has become desirable to find a sensing device that can be used with a variety of rotor tooth or aperture spacings and provides a high signal-to-noise ratio.